Change Alpha Blend Mode in WPF?

The System.Drawing.Graphics class has a property CompositionMode with two options: SourceOver (which, based on the alpha component, blends whatever is drawn with the background already existing) or SourceCopy which simply overwrites the background with whatever is being drawn.

Does something similar exist in WPF?

In WPF when i draw a PolyLine for example on top of another the new PolyLine always alphablends with 开发者_如何学Cthe background. I think that is independent of the container being used. I am using a Canvas but could not find a blend mode property anywhere. What I want to do is what the SourceCopy compositionmode mentioned above does. I.e. the new PolyLine should simply overwrite whatever is already on the Canvas.

Is there a simple way to do that, short of using pixel shaders (which - as far as I understand - wouldn't work anyways because I don't have access to the Canvas backbuffer).

I am not stuck with a Canvas and would be happy to use any container that supports overwrite mode.

I currently have a solution based on a WriteableBitmap for which I obtain a System.Drawing.Graphics context and then manipulate the CompositionMode. It works but since my window is fullscreen that solution has serious performance impacts.

Clarification and example: The WPF window is fully transparent and so is the Canvas (back ground color(0,0,0,0)). Now I draw a PolyLine with a Color.FromArgb(128,128,0,0). I now have a semi-transparent red polyline. Next I draw the same PolyLine with Color.FromArgb(0,0,0,0). The result is the same as before because of the alpha blending taking place. What I want, however, is that the red polyline is erased with the second polyline (which is exactly what the SourceCopy mode in the Graphics class does.

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I think all you need to do is make sure that the brushes used to fill/stroke the PolyLine have fully opaque alpha values (i.e. 255). Then the background shouldn't be blended into it.

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You could apply a Clipping Mask, this way you can provide the path to clip over the elements that are below it, but it might be tough to maintain after a lot elements are required to be clipped...